This disclosure relates to fuser or fixing members, and processes for making such fuser and fixing members. In particular, this disclosure relates to processes for making such fuser and fixing members, or other members, which are induction heated and where at least a layer of the member includes ferromagnetic or magnetic particles that enable induction heating of the member. This disclosure also relates to developing apparatuses using such fusing and fixing members.
In a typical electrostatographic printing apparatus, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles, which are commonly referred to as toner. The visible toner image is then in a loose powdered form and can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support, which may be a photosensitive member itself or other support sheet such as plain paper, transparency, specialty coated paper, or the like.
The use of thermal energy for fixing toner images onto a support member is well known. In order to fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
Typically, thermoplastic resin particles are fused to the substrate by heating to a temperature of between about 90° C. to about 160° C. or higher, depending upon the softening range of the particular resin used in the toner. It is not desirable, however, to raise the temperature of the substrate substantially higher than about 200° C. because of the tendency of the substrate to discolor at such elevated temperatures particularly when the substrate is paper.
Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means, including a roll pair maintained in pressure contact, a belt member in pressure contact with a roll, and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles generally takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art, and they can be adjusted to suit particular machines, process conditions, and printing substrates.
During operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the support are passed through a nip formed between the roll pair, or plate and/or belt members. The concurrent transfer of heat and the application of pressure in the nip effect the fusing of the toner image onto the support. It is important in the fusing process that no offset of the toner particles from the support to the fuser member takes place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus, increasing the background or interfering with the material being copied there. The so called “hot offset” occurs when the temperature of the toner is raised to a point where the toner particles liquefy and a splitting of the molten toner takes place during the fusing operation with a portion remaining on the fuser member.
The hot offset temperature or degradation of the hot offset temperature is a measure of the release property of the fuser roll, and accordingly it is desired to provide a fusing surface that has a low surface energy to provide the necessary release. To ensure and maintain good release properties of the fuser roll, it has become customary to apply release agents to the fuser members to ensure that the toner is completely released from the fuser roll during the fusing operation. Typically, these materials are applied as thin films of, for example, silicone oils to prevent toner offset. In addition to preventing hot offset, it is desirable to provide an operational latitude as large as possible. By operational latitude it is intended to mean the difference in temperature between the minimum temperature required to fix the toner to the paper, the minimum fix temperature, and the temperature at which the hot toner will offset to the fuser roll, the hot offset temperature.
Generally, fuser and fixing rolls are prepared by applying one or more layers to a suitable substrate. For example, cylindrical fuser and fixer rolls are typically prepared by applying an elastomer or a fluoroelastomer layer, with or without additional layers, to an aluminum core. The coated roll is then heated in a convection oven to cure the elastomer or fluoroelastomer material. Such processing is disclosed in, for example, U.S. Pat. Nos. 5,501,881, 5,512,409 and 5,729,813, the entire disclosures of which are incorporated herein by reference.
In use, important properties of the fuser or fixer members include thermal conductivity and mechanical properties such as hardness. Thermal conductivity is important because the fuser or fixer member must adequately conduct heat, to provide the heat to the toner particles for fusing. Mechanical properties of the fuser or fixer member are important because the member must retain its desired rigidity and elasticity, without being degraded in a short period of time. In order to increase the conductivity of the fuser or fixer members, it has been conventional to add quantities of conductive particles as fillers, such as metal oxide fillers. In order to provide high thermal conductivity, the loading of the filler must be high. However, increasing the loading of the filler tends to adversely affect mechanical properties of the coating layer, making the member harder and more prone to wear. For example, conventional metal oxides such as aluminum, iron, copper, tin, and zinc oxides are disclosed in U.S. Pat. Nos. 6,395,444, 6,159,588, 6,114,041, 6,090,491, 6,007,657, 5,998,033, 5,935,712, 5,679,463, and 5,729,813. These metal oxide filler materials, at loadings up to about 60 wt %, provide thermal conductivities of from about 0.2 to about 1.0 Wm−1K−1. However, the loading amount of the filler must be limited due to the increased hardness provided by high loading levels.
A problem with conventional fusing members, however, is the high thermal conductivity mismatch between the substrate layer and the outer layer. Heat generation in conventional fusing subsystems is generally accomplished by using heaters inside the fuser member, such as lamps located inside the fuser roll. In these subsystems, the centrally located lamps heat the fuser core, which is usually a metal core, which then transfers the heat to the coated layer(s) of elastomer, thermoplastic, or the like. Thus, in order to heat the outer surface of the fuser roll, the centrally located lamps must first heat the fuser roll substrate to a high temperature, and that heat must subsequently be transferred through the substrate and through the relatively thick applied outer layers, to reach the fuser member surface. However, the organic material coating layers applied over the substrate have orders of magnitude lower thermal conductivity values as compared to the metal substrate layer, thereby significantly limiting the heat transfer rate from the substrate layer to and through the organic outer layers. This limited heat transfer rate also results in poor temperature uniformity on the outer surface of the fuser member, particularly in running papers of different widths. Another problem with the low thermal conductivity coatings is the surface temperature drop and fluctuation of the heat roll in multi-page print runs.
One approach to address the above problems, is to use inductive heating of the fuser member layers. For example, a modified fuser member has been proposed that utilizes an inductive heating and heat pipe approach, where one end of fuse roll is heated, and that heat is transferred longitudinally down the length of the member by a heat pipe. This approach simplifies the geometry of the fuser subsystem, and helps to solve the problems of temperature non-uniformity and warm-up time. Heating is primarily accomplished through the highly thermally conducting heat pipe. However, the relatively low thermal conductivity of the outer organic layers still poses a barrier to heat transfer, particularly in thick, multi-layer coatings.
U.S. Pat. No. 6,078,781 discloses a fixing device that includes a first roller that is made of a conductive material and is rotated and driven; a second roller that is in contact with the first roller in the pressed state; and an induction heating unit that is arranged at the first roller side and concentrates the induction heating to the nip portion of the first roller. The induction heating unit of the fixing device is made of a high permeable material, has a core that is open at the surface opposite to the first roller and a coil wound round the core and generates magnetic flux on the core when high frequency current is supplied to the core and has a high projecting portion so that a part of the core closes the first roller.
Accordingly, there is a need in the art for improved filler materials for fuser and fixer members. Specifically, there is a need for improved filler materials that will provide higher thermal conductivity, but of a type or at loading levels that provide lower hardness to the member. There is also a need for improved filler materials that improve other mechanical properties of the member, such as longer life performance.